The cell groups of the ventral lateral lemniscus (VLL) transmit information from one ear to the contralateral inferior colliculus (IC). Although they are a major component of the central auditory system in all mammals, their extreme hypertrophy and elegance of organization in echolocating bats provides a unique opportunity to discover the function of each component cell group in a highly differentiated system. The VLL was long regarded as a single anatomical and functional region, but recent evidence shows that it contains at least three readily distinguishable cell groups. These are part of a larger system of parallel pathways which converge at the IC. A likely hypothesis is that each individual pathway is specialized for the analysis of some specific feature of sound, with the outputs being integrated at the IC. The long-term goal of the proposed research is to determine which features of sound are being analyzed and to elucidate the anatomical mechanisms for sound analysis in each of the nuclei of the VLL. The specific studies proposed are: l) To identify and characterize the subdivisions of the VLL on the basis of their cytoarchitecture and immunohistochemical reactivity. This step is preliminary to the remaining experiments. 2) To determine the tonotopy in each division. The presence of a complete tonotopic representation within a cytoarchitectural division would confirm that it is, indeed, an independent nucleus. 3) To map the pattern of afferent inputs, including terminal types, in each nucleus, and to identify the location and types of cells that project there. If it can be established that each VLL nucleus receives projections from a different subset of cells having known response properties, this will represent the first step in determining their input/output functions, and in understanding the ways in which auditory information is transformed at this level for relay to the inferior colliculus. 4) To relate the anatomical findings with the physiological response properties of cells in each division of the VLL. The proposed studies should provide the initial clues for establishing the function of each of the component nuclei of the VLL. In addition, they should reveal what type of information is being transmitted to the midbrain for integration with the outputs of other more thoroughly studied brainstem auditory centers, such as the superior olivary complex. The results may, in addition, provide a basis for understanding the early steps in analyzing sequences of sounds such as speech.